1. Field of the Invention
The present invention relates to a gas-pocket distributor assembly for distributing a mixture of liquid hydrocarbons and hydrogen-containing gas(es). More particularly, the present invention provides for an improved gas-pocket distributor assembly, a reactor containing the improved gaspocket distributor assembly, and a method for hydroprocessing a hydrocarbon feed stream.
2. Description of the Prior Art
Hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well known method of catalytically treating such heavy hydrocarbons to increase their commercial value. "Heavy" hydrocarbon liquid streams, and particularly reduced crude oils, petroleum residua, tar sand bitumen, shale oil or liquified coal or reclaimed oil, generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organo-metallic compounds which tend to deactivate catalyst particles during contact by the feed stream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the range of 212 degree(s) F. to 1200 degree(s) F. (100 degree(s) to 650 degree(s) C.) at pressures of from 20 to 300 atmospheres. Generally such hydroprocessing is in the presence of catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other metallic element particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurization, hydrogenitrification, hydrocracking, etc., of heavy oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one or more promoter(s) or catalytically active metal(s) (or compound(s)) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application.
Because these reactions must be carried out by contact of a hydrogen-containing gas with the hydrocarbon feed stream at elevated temperatures and pressures, the major costs of such processing are essentially Investment in vessels and associated furnaces, heat exchangers, distributor plate assemblies, pumps, piping and valves capable of such service and the replacement cost of catalyst contaminated in such service, and the cost of assembling the equipment. Commercial hydroprocessing of relatively low cost feed stocks such as reduced crude oils containing pollutant compounds, requires a flow rate on the order of a few thousand up to one hundred thousand barrels per day, with concurrent flow of hydrogen at up to 10,000 standard cubic feet per barrel of the liquid feed. Vessels capable of containing such a reaction process are accordingly cost-intensive both due to the need to contain and withstand corrosion and metal embrittlement by the hydrogen and sulfur compounds, while carrying out the desired reactions, such as demetallation, denitrification, desulfurization, and cracking at elevated pressure and temperatures. Pumps, piping and valves for handling fluid streams containing hydrogen at such pressures and temperatures are also costly, because at such pressures seals must remain hydrogen impervious over extended service periods of many months. It is also cost-intensive to insure that all of the equipment including distributor plate assemblies are assembled and/or manufactured correctly. It is important in such prior art distributor plate assemblies that they are essentially perfectly level to immunize flow distribution of the hydrocarbon feed streams from their sensitivity to the levelness of the distributor plate assembly.
As particularly distinguished from prior known methods of hydroprocessing, the method and apparatus in U.S. Pat. No. 5,076,908 to Stangeland et al more specifically provides a system wherein downward plug-flow of the catalyst bed is maintained over a wide range of counterflow rates of a hydrocarbon feed stream and hydrogen gas throughout the volume of the substantially packed catalyst bed. Such packed bed flow maintains substantially maximum volume and density of catalyst within a give vessel's design volume by controlling the size, shape and density of the catalyst so that the bed is not substantially expanded at the design rate of fluid flow therethrough.
The prior art does not disclose or suggest the above enumerated and pertinent features of either the total system or significant portions of such a system in U.S. Pat. No. 5,076,908 to Stangeland et al, as disclosed by the following patents:
Jacquin et al U.S. Pat. No. 4,312,741, is directed toward a catalyst replacement method in a hydroprocessing system by controlling the feed of hydrogen gas at one or more levels. Catalyst, as an ebullated bed counterflows through the reactor but is slowed at each of several levels by horizontally constricted areas which increase the hydrogen and hydrocarbon flow rates to sufficiently locally slow downward flow of catalyst. While local recycling thus occurs at each such stage, rapid through-flow of fresh catalyst, with resultant mixing with deactivated or contaminated catalyst, is suppressed. The ebullating bed aids simple gravity withdrawal of catalyst from the vessel. Improvement of the disclosed system over multiple vessels with valves between stages is suggested to avoid the risk of rapid wear and deterioration of valve seals by catalyst abrasion.
Kodera et al, U.S. Pat. No. 3,716,478, discloses low linear velocity of a mixed feed of liquid and H.sub.2 gas to avoid expansion (or contraction) of catalyst bed. By low linear velocity of fluid upflow, gas bubbles are controlled by flow through the packed bed, but the bed is fluidized by forming the bottom with a small cross-sectional area adjacent the withdrawal tube. This assists discharge of catalyst without backmixing of contaminated catalyst with fresh catalyst at the top of the single vessel. The range of bed level in the vessel is from 0.9 to 1.1 of the allowable bed volume (.+-.10%) due to fluid flow through the bed. A particular limitation of the system is that flow of the fluids undergoing catalytic reaction is restricted to a rate that will not exceed such limits, but must be adequate to ebullate the bed adjacent the catalyst withdrawal tube. Alternatively, injection of auxiliary fluid from a slidable pipe section is required. The patentees particularly specify that the diameter of the lower end of the vessel is smaller to increase turbulence and ebullation of catalyst adjacent the Inlet to the catalyst withdrawal line. Fluidization of catalyst is accordingly indicated to be essential to the process. However, the disclosed gas flow rates are well below commercial flow rates and there is no suggestion of temperatures or pressures used in the tests or the size, density or shape of the catalyst.
Bischoff et al, U.S. Pat. No. 4,571,326, is directed to an apparatus for withdrawing catalyst through the center of a catalyst bed counterflowing to a liquid hydrocarbon and gas feed stream. The system is particularly directed to arrangements for assuring uniform distribution of hydrogen gas with the liquid feed across the cross-sectional area of the bed. Such uniform distribution appears to be created because the bed is ebullating under the disclosed conditions of flow. Accordingly, considerable reactor space is used to initially mix the gas and hydrocarbon liquid feeds in the lower end of the vessel before flowing to other bottom feed distributors. The feeds are further mixed at a higher level by such distributor means in the form of "Sulzer Plates" or a "honeycomb" of hexagonal tubes beneath a truncated, conical, or pyramidal-shaped funnel screen. The arrangement may include an open ramp area parallel to the underside of the screen between the tube or plate ends. Further, to maintain gas distributions along the length of the catalyst bed, quench gas is supplied through upflowing jets in star-shaped or annular headers extending across middle portions of the vessel. The arrangement for withdrawal of spent catalyst requires ebullation of at least the lower portion of the bed. As noted above, added vessel space for uniform mixing of hydrogen and feed before introducing the fluids into an ebullated bed, as well as an ebullating bed, increases the required size of the hydroprocessing vessel, increases catalyst attrition, increases catalyst bed mixing and substantially increases initial, and continuing operating costs of the system.
Bischoff et al, U.S. Pat. No. 4,639,354, more fully describes a method of hydroprocessing, similar to U.S. Pat. No. 4,571,326, wherein similar apparatus obtains uniform ebullation through the vertical height of a catalyst bed, including a quench gas step.
Meaux U.S. Pat. No. 3,336,217, is particularly directed to a catalyst withdrawal method from an ebullating bed reactor. In the system, catalyst accumulating at the bottom of a vessel and supported on a flat bubble-tray may be withdrawn through an inverted J-tube having a particular ratio of the volume of the short leg of the J-tube to the longer leg. The diameter of the J-tube is suited only to flow of catalyst from a body of catalyst ebullated by the upflowing hydrocarbon feed and gas.
U.S. Pat. Nos. 4,444,653 and 4,392,943, both to Euzen, et al, disclose removal systems for catalyst replacement in an ebullating bed. In these patents, the fluid charge including hydrocarbon containing gas is introduced by various arrangements of downwardly directed jets acting laterally against or directly onto the conical upper surface of the bed support screen or screens. Alternatively, the feed is introduced through a conical screen after passing through a distributor arrangement of tortuous paths or a multiplicity of separate tubes to mix the gas and liquid feed over the conical screen. Such arrangements use a considerable volume of the pressure vessel to assure such mixing.
U.S. Pat. Nos. 3,730,880 and 3,880,569, both to Van der Toorn, et al, disclose a series of catalytic reactors wherein catalyst moves downwardly by gravity from vessel to vessel through check valves. As noted above, such valves require opening and closing to regulate the rate of flow, or to start and stop catalyst transfer, with catalyst in the valve flow path. Feed of process fluids is either co-current or countercurrent through the catalyst bed.
Van ZijllLanghaut et al, U.S. Pat. No. 4,259,294, is directed to a system for continuous or periodic replacement of catalyst by entrainment of the catalyst in oil pumped as a slurry either to withdraw catalyst from or to supply fresh catalyst to, a reactor vessel. Reacting feed is suggested to be either co-current or countercurrent with catalyst flow through the reactor. Valves capable of closing with catalyst in the line, or after back-flow or slurry oil, are required to seal off the catalyst containing vessel at operating temperatures and pressures from the receiving reactor vessel, or isolate the catalyst receiving lock hopper from the withdrawal section of the vessel.
Carson, U.S. Pat. No. 3,470,900, and Sikama, U.S. Pat. No. 4,167,474, respectively illustrate multiple single bed reactors and multi-bed reactors In which catalyst is replaced either continuously or periodically. The feed and catalyst flow co-currently and/or radially. Catalyst is regenerated and returned to the reactor, or disposed of. No catalyst withdrawal system is disclosed apart from either the configuration of the internal bed support or the shape of the vessel bottom to assist gravity discharge of catalyst.
One of the basic principles and teachings of Stangeland et al in U.S. Pat. No. 5,076,908, is that by specifically selecting the size, shape, and density of the catalyst pellets, combined with appropriate control of process liquid and gas velocities, random motion and backmixing of the catalyst can be minimized, and plugflow characteristics of the catalyst downward and the liquid and gas flow upward, maximized. Stangeland et al economically utilizes space within a hydroprocessing vessel over a wide range of processing rates without substantial random motion or ebullation of a packed bed of catalyst during high counterflow rates of the hydrocarbon feed and a hydrogen containing gas through the packed bed, while maintaining continuous or intermittent replacement of catalyst for plug-like flow of the bed through the vessel. Such plug flow with high processing rates is obtained by Stangeland et al by selecting the size, shape and density of the catalyst particles to prevent ebullation and bed expansion at the design flow rate so as to maximize the amount of catalyst in the vessel during normal operation and during catalyst transfer. Catalysts are selected utilizing data gained while studying catalyst bed expansion, such as in a large pilot plant run, with liquid hydrocarbon, hydrogen and catalyst at the design pressures and flow velocities within the available reaction volume of the vessel. Catalyst is removed from the bed by Stangeland et al through laminar flow of the catalyst particles in a liquid slurry system in which the liquid flow line is uniform in diameter, and substantially larger than the catalyst particles, throughout the flow path between the reactor vessel and a pressurizable vessel including passageways through the flow control valves.
However, the method and apparatus disclosed by Stangeland et al in U.S. Pat. No. 5,076,908, as well as the methods and apparatus(es) taught by the above-identified prior art patents relating to U.S. Pat. No. 5,076,908 to Stangeland, et al, all teach and/or suggest a distributor plate assembly that should be essentially level in a reactor vessel containing the distributor plate assembly. Flow distribution of a hydrocarbon feed stream passing through a distributor plate assembly is sensitive to the levelness of the distributor plate assembly within the reactor vessel. Even for a perfectly level plate, gas flow would pulse because of sloshing and varying liquid level. If the distributor plate assembly is not level, distribution of the hydrocarbon feed stream throughout an associated catalyst bed within the reactor vessel is affected. Also, these prior art hydroprocessing methods and apparatuses are saddled at times with hydrogen-containing gas bubbles that are too large, which also could affect the distribution of the hydrocarbon feed stream throughout a catalyst bed within the reactor vessel. Therefore, what is needed and what has been invented is a method and an apparatus or distributor assembly that is capable of producing an excellent, steady and smooth flow of a mixture of a gas, (e.g. a hydrogen-containing gas) and a liquid (e.g. a liquid hydrocarbon) into a chamber (e.g. a plenum chamber) without the indicated deficiencies associated with the prior art methods and apparatuses.